In order for the diesel to reach a supercritical state, Anitescu’s fuel system has first to heat it to around 450 degrees Celsius at a pressure of about 60,000,000 Pascal. Achieving the pressure is not a problem, Anitescu says, but increasing the temperature is more demanding.
Because fuel systems usually operate at temperatures below 80 degree Celsius, Anitescu and his colleagues used the heat from the engine’s exhaust to raise the fuel’s temperature. This causes further complications. “You need to prevent it from coking,” he says. Coking occurs when hydrocarbons in the fuel react, producing sticky deposits that can lead to fuel-system failures. The phenomenon can be avoided by diluting the fuel with an additive, such as carbon dioxide or water. In the Syracuse engine, a small amount of exhaust gas is introduced to act as an anticoking agent, a technique known as exhaust-gas recirculation.
The system has only been tested in a laboratory setup, but a prototype could be ready for testing by the end of the year, says Anitescu. The fuel system is designed to use conventional fuel injectors, even though these are designed to work with regular fluids. Anitescu says it may be possible to improve the performance by switching to a fluid state just below supercritical. This may allow vaporization to occur while getting better performance out of the injectors. “We have many options here,” he says.
At the same conference, Transonic Combustion, a company based in of Camarillo, CA, presented details of an alternative way to use supercritical fuels that involves a novel fuel injector and redesigning the engine’s entire valve system and combustion chamber.
But with either approach, going supercritical does not come without a cost, says Birgel. “You still need the viscosity because most diesel fuel systems depend upon the fuel for lubrication,” he says.
“This is an issue which has yet to be addressed,” admits Anitescu. He says it may be possible to introduce lubricants, but this would only be necessary in the final stage of the fuel system, where the fluid is at its hottest. For subcritical fuels, it may not be an issue, he says.